You are, absolutely, a turtle keeper after my own heart and the old saying, "If it ain't broke, don't fix it" certainly applies in your case. If, after all these years, that turtle is in as good shape you say it is, the only thing I would change is a bigger habitat and a platform of some sort for it to be able to get completely out of the water and dry off. You might let it have a few hours out in the sun also.
If the front claws look like 'Edward Scissorhands' ( really long) and the tail is fat close to the back shell edge, then you have a male. If the front claws are fairly short and the tail is narrow at the back shell edge and kinda long and the vent is also close to the back shell edge, you have a female.
You might also try Romain or Red Leaf Lettuce as a turt that old would eat vegetable matter ( lilly pads, underwater weeds,etc. ) as a main diet. Flies are just fine and if you have access to small frogs( Bull or Leopard ), your turt would just love one now and then. I had frogs that came out of the woods and took up residence in my little preformed 265 gallon pond and when one would try to swim in the pond over to the other side or climb up on a lilly pad, my turtles would ambush it and eat it with great gusto. I had a couple of big Koi in the pond also and they would snatch the frog out of the turts jaws and proceed to eat it when ever they could. Also had a pair of Garter Snakes that lived under the rocks on my waterfall and you should have seen them together come slithering down the watercourse and over the foot high fence where they would split up and go around each side of the pond and each grab a frog. What a sight that was! Don't start heating the habitat water as your turt does just fine in the water it's used to. You are one blessed person to have had a turtle for this many years and too radical a change in it's life would not be a good idea. I'm so glad you are on this forum. George
